As part of our continuing studies of the origin of Colorado tick fever virus within erythrocytes we have been examining hemopoietic liver cells obtained from mice infected in utero or when newborn for the presence of virions within mitotic erythroblasts and within erythroblasts undergoing the process of enucleation. Viral inclusions composed of fibrillar material and containing a few viral particles were seen within three different mitotic cells. An exhaustive study in search of viruses within erythroblasts undergoing the process of enucleation, however, has thus far been unsuccessful. We believe that our inability to demonstrate virions within erythroblasts undergoing nuclear expulsion is due primarily to the low number of such cells and to the rapidity in which the process of nuclear extrusion takes place. BIBLIOGRAPHIC REFERENCES: Oshiro, L.S., Levy, J.A., Riggs, J.L. and Lennette, E.H. Distinction between envelope antigens of murine xenotropic and ecotropic type C viruses by immunoelectron microscopy. J. Gen. Virol. 35: 317-323, 1977.